glXCreateGLXPixmap
creates an off-screen rendering area and returns its XID. Any GLX rendering
context that was created with respect to vis can be used to render into
this off-screen area. Use glXMakeCurrent to associate the rendering area
with a GLX rendering context.

The X pixmap identified by pixmap is used
as the front left buffer of the resulting off-screen rendering area. All
other buffers specified by vis, including color buffers other than the
front left buffer, are created without externally visible names. GLX pixmaps
with double-buffering are supported. However, glXSwapBuffers is ignored by
these pixmaps.

Some implementations may not support GLX pixmaps with direct
rendering contexts.